1. Field of the Invention
The present invention relates to an automatic document feeder arranged openable and closeable relative to an image reading unit and an image forming apparatus provided with such automatic document feeder, and more particularly to an automatic document feeder capable of reading a document in a reading posture set when closed and an image forming apparatus provided with such automatic document feeder.
2. Description of the Related Art
The vicinity of an automatic document feeder 100 used in an image forming apparatus is constructed as shown in FIG. 8. Specifically, the automatic document feeder 100 is openably and closeably arranged on an upper part of an image forming apparatus main body 110 where an image reading unit is built in. A lower surface 101 of the automatic document feeder 100 faces an upper surface 111 of the image forming apparatus main body 110 when the automatic document feeder 100 is closed. The automatic document feeder 100 includes a document placing portion 102 on which documents to be read are placed and a document discharging portion 103 located below the document placing portion 102. The automatic document feeder 100 supplies, in the closed state, a document placed on the document placing portion 102 to a narrow document reading surface (contact glass) 112 provided in the upper surface 111 of the image forming apparatus main body 110 for automatically reading the document and discharges the document to the document discharging portion 103. A wide document placing surface (contact glass) 113 is provided separately from the document reading surface (contact glass) 112 in the upper surface 111 of the image forming apparatus main body 110 so that a document can be read without using the automatic document feeder 100.
In the automatic document feeder 100 so constructed, if a thick document such as a book is left on the document placing surface 113 of the image forming apparatus main body 110, the lower surface 101 of the automatic document feeder 100 is lifted up from the upper surface 111 of the image forming apparatus main body 110, causing a problem of a paper jam. This is described in detail below. As shown in FIG. 9, a document fed to the document reading surface 112 and then to be read is discharged to the document discharging portion 103 via two scoop-up guides 114, 104 disposed near the document reading surface 112. The scoop-up guide 114 is disposed in the image forming apparatus main body 110, and the scoop-up guide 104 is disposed in the automatic document feeder 100. Thus, if the lower surface 101 of the automatic document feeder 100 is lifted up from the upper surface 111 of the image forming apparatus main body 110, a bottom end 104a of the scoop-up guide 104 comes to be located higher than an upper end 114a of the scoop-up guide 114. As a result, the document guided by the scoop-up guide 114 slips into a space under the scoop-up guide 104 to cause a paper jam.
Accordingly, it can be thought to prevent the occurrence of a paper jam by detecting the above lifting of the automatic document feeder 100. One of known technologies applicable to detect the lifting is such that an open/close sensor is disposed near a hinge openably and closeably supporting an automatic document feeder relative to an image forming apparatus main body and the operation of the automatic document feeder is stopped when it is detected by the open/close sensor that the automatic document feeder is lifted without being completely closed (see, for example, Japanese Unexamined Patent Publication No. 2005-184625).
By applying the above technology to the detection of the lifting, the occurrence of a paper jam resulting from a book left behind can be prevented.
However, since the open/close sensor is disposed near the hinge where a height change of the automatic document feeder relative to an angular change thereof is small, accuracy in detecting the lifting of the automatic document feeder is low. Thus, if even a thin book which would not cause any paper jam is left on the document placing surface of the image forming apparatus main body, there is a possibility of stopping the operation of the automatic document feeder, which makes the image forming apparatus inconvenient.
In the above Japanese Unexamined Patent Publication No. 2005-184625, it is also disclosed to use an angle sensor in addition to the open/close sensor. However, the use of the angle sensor is not enough to cope with a situation where such a thin book as not to cause any paper jam is left behind. In other words, it is difficult to detect the lifting of the automatic document feeder by the angle sensor since an angular change of the automatic document feeder is too small at a height to which the automatic document feeder is lifted up when such thin book is left behind.